TITLE: GCN CIRCULAR NUMBER: 29965 SUBJECT: Fermi-LAT Gamma-ray Observations of IceCube-210503A and detection of a new gamma-ray source, Fermi J0931.9+3633 DATE: 21/05/07 22:23:44 GMT FROM: Simone Garrappa at DESY S. Garrappa (DESY-Zeuthen), S. Buson (Univ. of Wuerzburg) and C. C. Cheung (Naval Research Laboratory) on behalf of the Fermi-LAT collaboration: We report an analysis of observations of the vicinity of the high-energy IC210503A neutrino event (GCN 29951) with all-sky survey data from the Large Area Telescope (LAT), on board the Fermi Gamma-ray Space Telescope. The IceCube event was detected on 2021-05-03 at 22:19:32.96 UT (T0) with J2000 position RA = 143.53 (+7.71, -5.16) deg, Decl. = 41.81 (+5.02, -5.68) deg (90% PSF containment). Seventeen cataloged gamma-ray (>100 MeV) sources are located within the 90% IC210503A localization region (4FGL-DR2, The Fermi-LAT collaboration 2020, ApJS, 247, 33). Based on a preliminary analysis of the LAT data over the timescale of 1-day prior to T0, none of these objects is significantly detected (> 5 sigma). We searched for intermediate (days to years) timescale emission from a new gamma-ray transient source. Preliminary analysis indicates no significant (> 5 sigma) new excess emission (> 100 MeV) at the IC210503A best-fit position.  Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC210503A best-fit position, the >100 MeV flux upper limit (95% confidence) is < 1e-10 ph cm^-2 s^-1 for ~12-years (2008-08-04 to 2021-05-03 UTC), and < 6.3e-9 (< 4.6e-8) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0. Based on a preliminary analysis, the cataloged object 4FGL J0920.9+4441 is significantly (> 5 sigma) detected in a 1-month integration time before T0. The source is associated with the FSRQ  S4 0917+44 (Pauliny-Toth et al. 1978, AJ, 83, 451) at redshift z = 2.18645 (Hewett and Wild 2010, MNRAS, 405, 2302) and is detected with flux (>100 MeV) = (1.8 +/- 0.8)e-08 ph cm^-2 s^-1 (30% of the 4FGL catalog value) and power-law index = 2.1 +/- 0.5, consistent with the 4FGL value of 2.35 +/- 0.01. The source has not shown enhanced activity within the last year, and it is not significantly detected at a timescale of 1-day before T0. Within the 90% confidence localization of the neutrino, 3.76 deg offset from the best-fit IC210503A position, a new excess of gamma rays, Fermi J0931.9+3633, was detected in an analysis of the integrated LAT data (100 MeV - 1 TeV) between 2008-08-04 and 2021-05-03. This putative new source is detected at a statistical significance >5 sigma (calculated following the prescription adopted in the 4FGL). Assuming a power-law spectrum, the candidate gamma-ray source has best-fit localization of RA = 142.97 deg, Decl. = 36.55 deg (5 arcmin 68% containment radius, 9 arcmin 99% containment radius) with best-fit spectral parameters, flux = (8.7 +/- 4.5)e-10 ph cm^-2 s^-1, index = 2.0 +/- 0.2. In a preliminary analysis of the LAT data over 1-day and 1-month prior to T0, Fermi J0931.9+3633 is not significantly detected in the LAT data. All values include the statistical uncertainty only. A possible counterpart of Fermi J0931.9+3633 is the BL Lac object SDSS J093209.60+363002.6 at RA=143.035791 deg, Dec=36.503676 deg, and redshift 0.15376 +/- 0.00017 (Anderson et al. 2007 AJ, 133, 313). This source is located about 4 arcmin from the Fermi J0931.9+3633 best-fit position, and within the gamma-ray 68% positional uncertainty. Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this source will continue. For these observations the Fermi-LAT contact persons are S. Garrappa (simone.garrappa at desy.de) and S. Buson (sara.buson at uni-wuerzburg.de). The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.